4x^4+12x³+5x²-6x-15
=4x^4-4x³+16x³-16x²+21x²-21x+15x-15
=(4x^4-4x³)+(16x³-16x²)+(21x²-21x)+(15x-15)
=4x³(x-1)+16x²(x-1)+21x(x-1)+15(x-1)
=(x-1)(4x³+16x²+21x+15)
=(x-1)[(4x³+10x²)+(6x²+15x)+(6x+15)]
=(x-1)[2x²(2x+5)+3x(2x+5)+3(2x+5)]
=(x-1)(2x+5)(2x²+3x+3).